With an electrophotographic copying apparatus according to the Carlson process, once the surface of a photoreceptor is electrified, an electrostatic latent image is formed by exposing, thereby the formed electrostatic latent image is developed by a toner, and then, the resultant positive image is transferred and fixed onto a paper sheet or the like. Additionally, the photoreceptor is subject to removal of remaining toner, electrical neutralizing and cleaning of the member surface, in order to allow prolonged repeated use.
Accordingly, the requirements for an electrophotographic photoreceptor include not only such electrophotographic properties as excellent electrification, excellent sensitivity and less dark decay, but physical properties in relation to repeated operations, such as a press life, wear-resistance and moisture-resistance, as well as satisfactory resistance to ozone generated when corona charge is applied, or to ultraviolet ray emitted during exposing (environmental resistance).
A conventional electrophotographic photoreceptor commonly used in the art is an inorganic photoreceptor having a photosensitive layer principally comprising a inorganic photoconductive material such as selenium, zinc oxide and cadmium sulfide.
Recently, the research and development of various organic photoconductive materials as materials for forming photosensitive layer in an electrophotographic photoreceptor is actively undertaken in the art.
For example, Japanese Patent Examined Publication No. 0496/1975 describes an organic photoreceptor having a photosensitive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone. This photoreceptor, however, does not necessarily meet the requirements for sensitivity and press life. To solve such a disadvantage, a development effort has been focused on an organic photoreceptor with high sensitivity and longer press life, wherein a photosensitive layer comprises a different materials capable of generating an electrical charge and of transporting the generated charge, respectively. In forming such a "separated function" type electrophotographic photoreceptor, a specific material intended for respective function is selected from a wide range of materials and, accordingly, an electrophotographic photoreceptor having aimed properties is rather readily available.
Diverse types of materials effectively serving as a charge generating material for such a separated function type electrophotographic photoreceptor have been proposed. One example of an inorganic material serving this purpose is an amorphous selenium described in Japanese Patent Examined Publication No. 16198/1968. This material is used together with an organic charge transporting material.
Additionally, many types of electrophotographic photoreceptors comprising an organic dye or organic pigment as a charge generating material have been proposed. For example, those having photosensitive layers containing a bisazo compound are known in the art by Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 37543/1972, 22834/1980, 79632/1979, and 116040/1981.
In electrophotographic process, however, ozone or another activated substance generated upon electrification by corona charge will cause such problems as deterioration of electrification properties, sensitivity loss and increase of residual potential.
Especially, of electrification properties and sensitivity loss as well as increase of residual potential are significantly promoted during repeated use, since the time of exposure to ozone and the other activated substances accumulatively increases.
To improve above properties, an antioxidant is added as described in Japanese Patent O.P.I. Publication Nos. 130759/1981, 73744/1982, and 122444/1981. Despite an antioxidant added, there still remain dilemmatic problems unsolved: photoreceptor favorable in electrification properties tends to have lower initial sensitivity; photoreceptor preferable in initial sensitivty is rather less resistant to deterioration due to repeated use; photoreceptor less liable to deterioration due to repeated use is insufficient in initial sensitivity and electrification properties.